1.0 Mobile Communication Systems
In a typical cellular radio system, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a radio access network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Packet System (EPS) have completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to radio network controller (RNC) nodes. In general, in E-UTRAN/LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes, e.g., eNodeBs in LTE, and the core network. As such, the radio access network (RAN) of an EPS system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
1.1 Heterogeneous Networks
There have been several proposals to meet the ever increasing traffic demands and high quality expectations from end users for mobile broadband services. The upgrading of the existing base stations to use higher data rate technologies such as High Speed Packet Access (HSPA) or Long Term Evolution (LTE), or use other optimizations such as Multiple Input Multiple Output (MIMO), antenna tilting, etc., is one of the most widely adopted means to meet these demands. This can be further enhanced by a straightforward increasing the number of base stations (eNBs) in the network, known as macro densification. However, these methods of improving the data rate can provide system gains only to a certain extent and they can end up being very expensive. As such, the concept of Heterogeneous Networks (HetNets), where the existing homogeneous network is overlaid with additional lower-power, low-complexity base stations, is currently being researched as a solution to mitigate the cost and/or capacity limitations of macro densification or upgrading.
The homogeneous layer of macro cells is known as a “macro” layer, as the eNBs in this layer have large coverage areas. The non-homogenous layer contains low power nodes such as “picos” (low power eNBs, for indoor or outdoor usage), “femtos” (home base stations (HeNBs), usually for indoor home usage) or relays (usually for coverage extension). Femtos that are open only to few users (within a household, a shop, etc.), are termed within 3GPP as Closed Subscriber Group (CSG). FIG. 1 illustrates an example HetNet deployment scenario.
HetNets are expected to offer a low cost alternative to macro densification and will more likely be effective as the deployment of the low power nodes can be made more focused towards the hot spots and areas with coverage problems. The term “small cell” is used to refer to a pico or a femto cell for the rest of this document.
1.2 Handover in LTE
Handover is one of the important aspects of any mobile communication system, where the system tries to assure service continuity of the User Equipment (UE) by transferring the connection from one cell to another depending on several factors such as signal strength, load conditions, service requirements, etc. The provision of efficient/effective handovers (minimum number of unnecessary handovers, minimum number of handover failures, minimum handover delay, etc.), would affect not only the Quality of Service (QoS) of the end user but also the overall mobile network capacity and performance.
In LTE, UE-assisted, network controlled handover is utilized. The network configures the UE to perform measurements and send measurement reports when certain criteria are met. Based on these reports the UE is moved, if required and possible, to the most appropriate cell that will assure service continuity and quality. A UE measurement report configuration comprises the reporting criteria (whether it is periodic or event triggered) as well as the measurement information that the UE has to report.
In LTE, the most important measurements metric used are the Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ). RSRP is a cell specific measure of signal strength and it is mainly used for ranking different cells for handover and cell reselection purposes, and it is calculated as the linear average of the power of the Resource Elements (REs) which carry cell-specific Reference Symbols (RSs). The RSRQ, on the other hand, also takes the interference into consideration by taking the total received wideband power into account as well.
One of the configuration parameters that UEs receive from their serving eNB is a parameter called “S_measure”, which tells the UE when to start measuring neighboring cells. If the measured RSRP of the serving cell falls below the S_measure, indicating the signal of the serving cell is not that strong anymore, the UE starts measuring the signal strength of RSs from the neighboring cells.
While the S_measure determines when the UE starts measuring other cells, there are several other measurement configuration parameters that specify the triggering of handover measurement reports from the UE. The following event-triggered criteria are specified for intra-Radio Access Technology (RAT) measurement reporting in LTE:                Event A1: Serving cell becomes better than absolute threshold.        Event A2: Serving cell becomes worse than absolute threshold.        Event A3: Neighbor cell becomes better than an offset relative to the serving cell.        Event A4: Neighbor cell becomes better than absolute threshold.        Event A5: Serving cell becomes worse than one absolute threshold and neighbor cell becomes better than another absolute threshold.        
The most common measurement report triggering event related to handover is A3; its usage is illustrated in FIG. 2. The triggering conditions for event A3 can be formulated as Expression (1):N>S+(Hs−CIOs,n)  Expression (1)where N and S are the signal strengths of the neighbor and serving cells, respectively, Hs is the hysteresis parameter that the serving cell applies for event A3, and CIOs,n is the Cell Individual Offset (CIO) set by the serving cell for that specific neighbor cell. If this condition is satisfied and it remains valid for a certain duration known as Time To Trigger (TTT), the UE sends a measurement report to the serving eNB (in FIG. 2, entry condition for event A3 is satisfied at point A and measurement report is sent at point B in time when both entry condition and TTT criteria are fulfilled, i.e., the event criteria is fulfilled). When the serving eNB gets the measurement report, it may initiate a handover towards a neighbor.
1.3 Mobility Robustness Optimization
As indicated above, handover in LTE is controlled via several parameters. Incorrect parameter settings can lead to several problems, such as (for example) Radio Link Failure (RLF) and Ping pong handover.
If the parameters are set in such a way that the UE does not report handover measurements on time and the UE loses the radio connection with the serving cell, the UE experiences a Radio Link Failure (RLF) before handover could be initiated. This is known as “Too Late HO” and the UE tries to re-establish the connection with another cell after the RLF detection timers have expired. On the other hand, if the parameters are set to trigger handover very early, RLF might occur shortly after handover preparation has been completed and the UE has either completed or it is in the process of completing it in the target cell. This is known as “Too Early HO” and the UE tries to re-establish the connection with the source cell after the RLF detection timers have expired. Even if the handover is triggered at the right time, incorrect settings of the CIO can make the UE to handover to the wrong cell, which is followed by a RLF and a re-establishment request in a cell other than the target cell or the source cell. This is known as “HO to a wrong cell”.
Improper handover parameter setting can make the UE handover back and forth between two neighboring cells, e.g., “Ping pong handover”. An example of this is a setting that makes the triggering conditions for the handover events (A3) valid between the source and neighbor cells at the same time.
When the UE receives a certain number of (N310) consecutive “out of sync” indications from the lower layer, it assumes a physical layer problem is ensuing, and a timer (T310) is started. If the UE does not receive a certain number of (N311) consecutive “in sync” indications from the lower layer before T310 expires, RLF is detected. RLF is also detected when random access problem is indicated from MAC or upon indication that the maximum number of RLC retransmissions has been reached.
Another type of failure is a HO failure (HOF). When the UE receives a HO command (i.e., RRCConnectionReconfigurationRequest with mobilityControlInfo), it starts a timer (T304), and if this timer expires before the HO is completed (i.e., RRCConnectionReconfigurationComplete message is sent by the UE), a HO failure is detected.
The less the number of RLFs and HOFs in the system, the better the performance, both from user and system perspective. As such, it is very desirable to configure the system parameters (the HO parameters discussed above being the main ones) appropriately.
Configuring all the HO parameters manually is too expensive and can be very challenging. As such, Mobility Robustness Optimization (MRO) has been introduced in 3GPP to automate the dynamic configuration of handover parameters. MRO tries to gather statistics on the occurrence of Too Late HOs, Too Early HOs and HO to the wrong cell, and these statistics are used to adjust the handover parameters such as Hysteresis, CIO and TTT.
For MRO, the different HO problems discussed above are communicated between neighboring cells in different ways. For Too Late Handovers, an RLF INDICATION message is sent via X2 from the eNB to which the UE tries to re-establish a connection to the eNB where the RLF occurred. The RLF INDICATION message contains:                PCI of the cell in which the UE was connected prior to RLF (known as failure cell).        ECGI of the cell where RRC re-establishment attempt was made.        UE Identity: C-RNTI and MAC ID of the UE in the failure cell        RLF report (in a UE RLF Report Container IE        
If an eNB receives an RLF INDICATION message from a neighbor eNB, and if it finds out that it has sent a UE CONTEXT RELEASE message towards that neighbor eNB within the last Tstore_UE_cntxt seconds (i.e., it means that very recently the concerned UE was handed over properly to it from the same eNB), the eNB responds by sending a HANDOVER REPORT message that indicates Too Early Handover.
If an eNB receives an RLF INDICATION message from a neighbor eNB, and if it finds out that it has sent a UE CONTEXT RELEASE message towards another neighbor eNB within the last Tstore_UE_cntxt seconds (i.e., it means that very recently the concerned UE was handed over properly to it from another eNB), the eNB responds by sending a HANDOVER REPORT message that indicates Handover to the Wrong Cell.
The HANDOVER REPORT message contains:                Type of detected handover problem (Too Early Handover, Handover to Wrong Cell);        ECGI of source and target cells in the handover;        ECGI of the re-establishment cell (in the case of Handover to Wrong Cell);        Handover cause (signaled by the source during handover preparation).        
Thus, by analyzing the received RLF INDICATION and HANDOVER REPORT messages within certain duration, eNBs can configure the optimal HO parameters to be used with their neighbors.
1.4 Enhanced Mobility Robustness Optimization
As mentioned above, current mechanisms such as Mobility Robustness Optimization (MRO) try to optimize mobility by fine tuning of mobility thresholds such as CIOs with the objective of preventing further failures from occurring.
Though the standardized MRO mechanisms can be very useful, they have limitations, specifically in HetNet scenarios because the cells involved in mobility are a mixture of large coverage layer cells and small hot spot coverage cells aimed at increasing capacity in a much localized way. Hence, the success of UE mobility in a HetNet scenario greatly depends on the time the UE is expected to spend on a given cell (i.e., how big the cell is and how fast the UE is moving). If this is not considered, UEs can experience lots of unnecessary handover (i.e., fast speed UE handed to a small sized neighbor and to be handed over after a very short duration to another neighbor) or even failures (e.g., if handover of a high speed UE to a small sized neighbor is initiated, target cell signal may become very weak by the time handover is completed and failure may occur).
Currently, UE speed information is used to adjust cell reselection (cell reselection thresholds) and handover parameters (TTT) to some extent. First the UE estimates its speed (high, medium, normal, known as the mobility state of the UE) based on MobilityStateParameters configuration received from the eNB, which relates the number of handovers in a given time to a certain mobility state. Thus, in the case of handover, the UE calculates its mobility state and adjusts the TTT accordingly by multiplying the TTT with a scaling factor associated with each mobility state. The drawbacks of this way of handover parameter adjustment, especially in a HetNet scenario are:                The speed calculation based on number of handover is not accurate even in a homogenous network, and it becomes even worse in a HetNet.        All neighbors are treated equally. For example, the UE adjusts the TTT to the same value whether the concerned target is a big macro cell or a very small pico cell.        
On top of that, only the TTT is adjusted in a speed dependent manner, and the same CIO is used regardless of the UE's speed, which can greatly undermine the benefit of the adjusted TTT.
From the above it can be deduced that it would be preferable not to handover UEs moving at high speeds to very small coverage cells due to the UE permanence in the cell being likely very short or even due to the high probability of mobility failures due to the target small cell's signal becoming very weak by the time the handover is completed.
Therefore, for UEs moving at high speed it would be advisable to select the handover target cell not only on the basis of the strongest received signal but also on the basis of the risk of handover failure if the target consists of a very small cell. Namely for these UEs the best target would be a wide coverage cell.
However, if source cell mobility parameters such as TTT are adjusted to allow high speed mobility to “jump” small cell targets and to select directly larger coverage cells, this would degrade the mobility performance of low speed UEs. These UEs in fact would need to be handed over to small coverage cells given that they might be able to camp on such cells for a long time while moving at slow speed.
U.S. Provisional Patent application 61/515,225, filed Aug. 4, 2011, entitled “Improved Handover Robustness in Cellular Radio Communications”, which is incorporated by reference herein in its entirety, proposes enhancement of current MRO mechanisms that address both the UE speed and the size of the target cell. Therein it is proposed to add to the X2 RLF INDICATION message the cell size of the cell originating the message. This, combined with UE speed information already contained in the UE RLF Report Container IE, allows the receiving node to adjust its target selection criteria based on UE mobility and neighbor cell size. Furthermore, it was also proposed to include the size of the cells involved in mobility and UE RLF Report Container IE in the X2 HANDOVER REPORT message sent from target eNB to source eNB. The source eNB thus becomes aware of UE measurements taken at the time of failure and size of neighboring cells, which allows the source to adjust its mobility thresholds towards neighbor cells and to prioritize certain targets depending on UE mobility. Differentiated target selection for high speed UEs are proposed where a per-UE, per target cell CIO is used or/and target prioritization at the source eNB is made based on previously monitored failure events.
1.5 Cell Size and its Impact on Random Access Operations in LTE
In Wideband Code Division Multiple Access (WCDMA), one of the predecessors of LTE, the Random Access Channel (RACH) is used for initial network access as well as for short message transmission. In LTE, the RACH use is limited only for initial network access, and is used primarily for the UE to acquire uplink time synchronization (in situations such as when the UE is handed over to a target cell, when it goes from IDLE mode to CONNECTED mode, or if it has lost the synchronization due to RLF). An uplink synchronized UE can also use the RACH for the purposes of sending a Scheduling Request (SR) if it doesn't have any other uplink resource allocated in which to send the SR.
Two kinds of RACH procedures are available in LTE, namely contention-based and contention-free. For contention-based RA, the UE randomly chooses a preamble, while a pre-assigned preamble is used for the contention-free cases. Contention free is usually the preferred method for time-critical cases such as synchronization during handover.
There are totally 64 preambles, a certain number of which are reserved for contention free access by the eNB. The ones available for contention-based access are broadcasted to all UEs, while contention-free ones are dedicatedly assigned on need basis.
Four Preambles formats as shown in table 1 are defined for FDD:
TABLE 1RA preamble formats in LTE.PreambleCyclicSequenceSub framesFormatPrefix (μs)duration (μs)requiredCell Range0103.138001up to 14 km1684.388002up to 77 km2203.1316002up to 29 km3684.3816003up to 100 km
Since actual physical transmission occurs in units of sub-frames (1 ms), the remaining time from the sum of the cyclic prefix and sequence duration determines how far away the UE can be without overlapping another UE's access attempt (the guard time) and hence the coverage.
While the RA preamble format determines the length of the preamble, the generation of the actual preamble is mainly determined by the RootSequenceIndex and ZeroCorrelationZoneConfig (which is the cyclic shift to be applied to the root sequence) parameters. Using these two parameters, it is possible to generate sequences that are orthogonal regardless of the delay spread and the time uncertainty of the UEs. Sixteen different cyclic shift values have been defined and the cyclic shift values are also related to the cell range and the relationship is shown in FIG. 3.
1.6 Measurements
1.6.1 UE Measurements
Most measurements in LTE are done by the UE on the serving as well as on neighbor cells over some known Reference Symbols (RS) or pilot sequences. The measurements are done for various purposes. Some example measurement purposes are: mobility, positioning, self organizing network (SON), minimization of drive tests (MDT), operation and maintenance (O&M or OAM), network planning and optimization etc. The measurements may also comprise of cell identification e.g. PCI acquisition of the target, CGI or ECGI acquisition of the target cell, system information acquisition of the target cell, be it an LTE cell or any inter-RAT cell.
Examples of mobility measurements in LTE are:                Reference symbol received power (RSRP)        Reference symbol received quality (RSRQ)        
Examples of well known positioning measurements in LTE are:                Reference signal time difference (RSTD)        RX-TX time difference measurement        
1.6.2 eNB Measurements
Some measurements may also require the eNB to measure the signals transmitted by the UE in the uplink. One important measurement performed by the eNB in LTE is the estimation of Timing Advance (TA). For LTE, uplink orthogonally is required to avoid intra-cell interference and as such it is important to have all the uplink signals to be time-aligned when they are received at the eNB. Thus, eNBs try to compensate for the propagation delays differences of their UEs (due to their differing distances from the eNB), by instructing them to apply different timing advances, and the UEs will apply the configured timing advance when they are transmitting. The TA can first be estimated during the initial random access procedure when the UE establishes a connection with the eNB (either due to handover or going from IDLE to connected mode). TA updates are then performed throughout the duration the UE is connected to the eNB, as the propagation delay might change, for example due to the movement of the UE, the change of the environment due to movement of other objects in a dense urban setting, etc. For these updates, the eNBs may measure received uplink signals such as Sounding Reference Signals (SRS), Channel Quality Indicator (CQI), ACKs and NACKs in response to downlink data reception, or the uplink data transmission. The details of uplink timing measurements at the eNB are not standardized and left to implementation.
eNBs that have multiple antenna elements could also use their diversity to measure the Angle of Arrival (AoA) of the uplink signals that receive from their UEs. The AoA and TA can be used to estimate the relative co-ordinates of the UEs within the cell.
2.0 Problems with Existing Solutions
As discussed above, including the cell size information in MRO reports can lead to a more accurate configuration of HO parameter that can improve system performance by reducing the occurrence of RLFs and HOFs in HetNets, regardless of the speed of the UEs in the system.
As also discussed above, some rough estimate of the cell size can be made from the random access preambles. However these estimates can only tell the maximum possible cell size that can use such a preamble. Also, the minimum cell size which can be guessed from these is about 800 m (see FIG. 3). As such, it will be impossible to distinguish a pico cell of 50 m radius with an LTE macro cell of 500 m radius because they can both use similar preamble formats and cyclic shifts.
Another problem in using the RA preambles is that the preambles are decided based on the overall coverage area (how much delay the UEs signal could experience). That means in the case of remote site deployments such as Distributed Antenna Systems (DAS), the length of the fibre to the remote cells must be considered as part of the cell radius. Since the speed of electromagnetic waves over fibre is only two thirds of the speeds in free space, the total cell radius reduces the values shown in Table 1.
In a homogenous network that has only macro cells, the cell size can be roughly estimated based on the transmission power used by the eNBs, and they are rather static. However, the effective cell size/shape is highly impacted by the environment. This is illustrated in FIG. 4, where a base station is deployed in a urban-like environment. The base station is installed at a four-way intersection, with a building at each corner. If there were no buildings, the size/shape of the cell would have been as the one depicted in the dashed lines. However, due to the big indoor propagation loss, the effective size/shape of the cell would be reduced to the grey area. Although the example of FIG. 4 shows the impact of static buildings, temporal changes in the environment caused by moving objects, seasonal changes (e.g. the falling of leaves, snow melting, etc.), construction of new buildings, etc. will also impact the effective cell size/shape. Thus, it is very challenging to obtain accurate cell size/shape information during the radio planning phase, and the effective cell size/shape can vary from time to time.
Performing handovers solely based on radio conditions can lead to sub-optimal results, since the cell shape as well as its overlapping region with its neighbours can be highly irregular. As shown in FIG. 5, due to the propagation environment, the three cells have varying shapes. If a UE is leaving cell A in the vicinity of region x, then cell B is the obvious handover candidate. On the other hand, in region Y, if a UE is handed towards cell B from cell A, it is very likely that soon it will have to be handed over to cell C. It might also be possible that the UE (especially if it is moving at high speed) will experience a failure soon after it has been handed over to B. The failure may occur because there may not be sufficient time to perform measurements, report them to eNB B and handover executed properly during the short period that the UE stays in cell B. A handover algorithm that is based only on radio conditions, without considering the neighbor cell shapes and overlapping region might always try to handover a UE from cell A to cell B, whether the UE is approaching cell B in region x or y, possibly causing unnecessary handovers, or even a connection failures.
In HetNets even having detailed information about the environment will not provide accurate cell size/shape information due to two main reasons: Cell Range Expansion (CRE) and the location of small cells relative the macro eNBs.
2.1 Cell Range Expansion
The big differences in the transmit powers of small cells and macros means the downlink coverage of the small cells is much smaller than the macros. However, in the uplink this is not the case because the uplink coverage depends on the received power at the eNBs (be it a pico/femto or macro), and as such it is similar for all eNBs and the uplink handover boundaries can be determined based only on the channel gain (i.e., selecting the node that provides minimum path loss). Thus, the uplink and downlink handover boundaries can be quite different in HetNets, as compared to the homogenous network case in which they are very similar. Due to the small size of pico/femto cells, the path loss between UE and Pico/Femto eNB is relatively small (provided the UE is in proximity of the Pico/Femto cell).
However, the reduced Tx power of such cell might mean that the UE is outside the pico/femto downlink coverage area. This is illustrated in FIG. 6.
If serving cell selection is based only on downlink received signal strength (i.e., RSRP) as in LTE release 8, most of the UEs will select the macros and the small cells may end up serving almost no UEs, which is against one of the primary reasons for installing small cells to begin with, i.e., sharing the load of the macros. Cell Range Extension (CRE) is the expansion of the coverage of the small cell in order to balance the load between the macros and small cells. This can be realized in several ways, for example:                Adjust the transmission power of the small cell depending on the load in the system        Use cell selection offsets and handover thresholds that favour the small cell        Adopting the use of Almost Blank Subframes (ABS) at overlaying macro cell, in order to reduce interference on such subframes (by preventing the macro from transmitting any data traffic) and therefore increase the radio quality of Pico/Femto cell signal to cover a larger area).        
With Cell Range Extension (CRE), the best uplink signal will be received by the best node (the small cell) in the expanded cell area, and thus some load is relieved from the macro. However, the downlink interference is high in the expanded cell area. Several advanced interference management techniques, such as interference cancellation at the UE and enhanced Inter-cell Interference Co-ordination (eICIC).
With regard to the reporting of cell sizes, Cell Range Extension (CRE) imposes a problem because the effective cell size of the small cells will be time varying.
2.2 Relative Location of Small Cells
The effective size of the pico cells is also dependent on the relative location of the small cells relative to the macro eNBs. For example, simulation results have shown (which are duplicated in Table 2) that the coverage area of the small cell can vary considerably, by more than threefold, when the small cell is placed 1.5 cell radius away from the macro eNB (where the macro eNB is situated at the far end of the cell) as compared with the case where the small cell is located 0.5 cell radius away.
TABLE 2ESTIMATES OF THE PICO COVERAGE AREA RADIUSFrequencyCell0.5x1x1.5xand ISDselectionradiusradiusradius2 GHz,RSRP 7 m14 m21 m500 m2 GHz,PL16 m33 m50 m500 m700 MHz,RSRP24 m49 m74 m1732 m700 MHz,PL57 m117 m 176 m 1732 m
Thus, two small cells using the same transmission power and range extension settings can end up having quite different effective cell sizes due to differences in their relative location from the macro, and as such it impacts the reporting of cell sizes that is addressed hereinbelow.